Cable television systems have been in widespread use for many years and extensive networks have been developed. The Data Over Cable Service Interface Specification (DOCSIS) is a cable modem standard used for transferring data over a cable TV network. An example of a broadband network commonly used by cable TV operators is shown in FIG. 1. Existing headend/CMTS architectures include an Edge quadrature amplitude modulation (QAM) device 102 at the headend or hub device, having multiple Gigabit Ethernet (GigE) input ports. The Edge-QAM device receives signals from the modular cable modem termination system (M-CMTS) 100 for downstream transmission, for example, re-packetizes the video or data into a transport stream, e.g., MPEG transport stream, and digitally modulates the transport stream onto a downstream RF carrier using QAM. The Edge-QAM device can communicate signals from the CMTS to network elements 103(1-N).
A DOCSIS timing interface (DTI) server 101 provides accurate reference clock information for Edge-QAM devices 102 and modular CMTS in a typical M-CMTS architecture via a DOCSIS timing interface (DTI). The DTI is the protocol defined in the DOCSIS 3.0 DTI specification, which aims to ensure that the Edge-QAM modulator and the M-CMTS device are synchronized. The DTI in the headend may address both jitter for multiple Edge-QAMs and provide for redundancy, e.g., by allowing for a switch-over to a redundant Edge-QAM without re-ranging. The DTI functions to provide the same signal to both the primary and redundant Edge-QAMs.
Using DTI is expensive. For example, incorporating the DTI requires an expensive master clock and a separate timing interface to both the CMTS and Edge-QAM. Further, existing mechanisms for addressing jitter for Ethernet links require technology installed on both the headend Edge-QAM and the CMTS. For example, the DTI is installed at the headend, where the Edge-QAM is separate from the CMTS, e.g., in separate chassis. The DTI clock is connected to each chassis to maintain accurate timing and jitter between the CMTS and Edge-QAM streams. Further, the SYNC from the DTI server does not support long distances. For example, the current standard for the DOCSIS timing interface is limited to 250 meters, and the DTIs developed to date are limited to supporting this distance (e.g., limited to less than 250 meters). An alternative to DTI in existing DOCSIS M-CMTS deployments would be desirable, both for cost savings and to work over longer remote distances.